Problem: Solve for $r$ : $r - 12 = -23$
Solution: Add $12$ to both sides: $ \begin{eqnarray} \\ r - 12 &=& -23 \\ \\ {+12} && {+12} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{-12 + r} &\hphantom{=}& \hphantom{-23} \\ r &=& -23 {+ 12} \\ \end{eqnarray} $ Simplify. $ r = -11$